A. Field of the Invention
This invention relates to the production of elemental sulfur from an H.sub.2 S-bearing gas stream.
B. Prior Art Background
The problems attendant to removal of H.sub.2 S, and SO.sub.2 and other sulfur-bearing gases from various gas streams has been long known and researched. Many of such processes are thoroughly discussed in a number of publications, including "Gas and Liquid Sweetening," Maddox, Second Edition, April 1977. There are several H.sub.2 S removal processes which result in relatively rich H.sub.2 S gas streams even when other conditions make the production of a rich H.sub.2 S stream difficult (e.g., large quantities of CO.sub.2 relative to H.sub.2 S in the original gas stream). An example of such a process is that discussed in U.S. Pat. No.3,989,811 issued Nov. 2, 1976. Most of such processes, however, remove much of the CO.sub.2 along with the H.sub.2 S and do not result in a highly concentrated H.sub.2 S stream.
Relatively rich H.sub.2 S-bearing streams have most commonly been converted to sulfur using the so-called Claus process. In the Claus process, the objective is to combust exactly one-third of the H.sub.2 S to SO.sub.2 and then react the two gases in the vapor phase in the presence of a suitable catalyst according to the following reaction: EQU 2H.sub.2 S+SO.sub.2 =3S.degree.+2H.sub.2 O
Controlling the air required to combust exactly one-third of the H.sub.2 S is extremely difficult, particularly when any quantities of hydrocarbons are present. In addition, since sulfur and water vapor are not removed from the reacting stream, the reaction does not proceed to completion. The equipment for a Claus plant is large and expensive, and conversion to sulfur is typically only 93% to 97%. Ultimate conversion almost never reaches 99%, even when using 4 or more catalytic reactors in series. Even at 99% conversion, in excess of 3000 parts per million of SO.sub.2 would be emitted to the atmosphere without a tail gas treating unit, many of which are known to those skilled in the art, and which are also expensive installations.
Very lean (10% or less) H.sub.2 S bearing gas streams are not easily handled by any sulfur conversion process, particularly the Claus process, because flame stability in the combustion of H.sub.2 S to SO.sub.2 is difficult or impossible to sustain at such low levels of H.sub.2 S.
There have been a number of processes proposed in which the H.sub.2 S-SO.sub.2 reaction is not carried out in the vapor phase as in the Claus process, but in a solvent, either organic or aqueous. An example of such a process utilizing an organic solvent is disclosed in U.S. Pat. No. 3,598,529. In many of such organic solvent processes, the vapor pressure of the solvent is sufficiently high that the contaminating gases, predominately N.sub.2 and CO.sub.2, carry off such an amount of solvent that the process is not economic. In addition, various studies have reportedly demonstrated that direct absorption by organic solvents leads to reaction with H.sub.2 S or SO.sub.2 to form complexes which can only be broken by tearing the solvent apart.
Aqueous medium absorption of SO.sub.2 and reaction with H.sub.2 S in that aqueous medium contribute to the formation of sulfates, thiosulfates and thionates. The formation of these "heat stable salts" leads to fouling of the absorbent and expensive processes must be utilized to rid the absorbent of them. Disposal of the heat stable salts in an ecologically satisfactory manner presents yet another difficultly with aqueous systems.